Keto devotees believe that if you banish most carbs (including fruit!) and embrace fat, you can lose weight without feeling hungry.

The diet’s long history in science also lends it credibility. Doctors have been prescribing ketogenic diets to treat epilepsy for nearly a century, and increasingly believe it may hold promise for people with Type 2 diabetes.

But what’s lost in the many trend articles and books about “going keto” for weight loss today is that this diet is the same one the now-late Dr. Robert Atkins and other low-carb evangelists have been selling since the 1960s. (Diet peddlers have an incredible knack for rebranding old ideas over and over, and in our eternal confusion about what to eat, we keep falling for it all.)

Those older keto diets didn’t work for most people hoping to slim down, and there’s no evidence the newly popular keto diet will be any different. Here’s why.

How the ketogenic diet works

To understand the ketogenic diet, you need a quick primer on how the human body gets energy. We are fueled primarily by glucose, or blood sugar, much of which we derive from carbohydrates in foods like bread, fruit, potatoes, and sweets.

If glucose levels in the blood drop to really low levels, we’d pass out and die. But, interestingly, the body can’t store much glucose — only enough to last a couple of days. So if we forgo eating carbs for a few days, we need other ways to keep going. One of those is a process called ketogenesis.

In ketogenesis, our livers start to break down fat into a usable energy source called ketone bodies, or ketones for short. “Organs like the brain that normally rely primarily on glucose for fuel can begin to use a substantial amount of ketones,” said Kevin Hall, a National Institutes of Health senior investigator who has studied the ketogenic diet. So ketones can stand in for glucose as fuel for the body when there’s a glucose shortage. “It’s an amazing physiological adaption to starvation that allows tissues like the brain to survive,” Hall added.

Once ketogenesis kicks in and ketone levels are elevated, the body is in a state called “ketosis.” There are a few ways to get into ketosis. One is through fasting: When you stop eating altogether for an extended period of time, the body will ramp up fat burning for fuel and decrease its use of glucose (which is part of the reason people can survive for as long as 73 days without food).

Another way to get into ketosis is by eating less than 20 to 50 grams of carbs — or a slice or two of bread — per day. So people on a ketogenic diet get 5 percent of their calories from carbohydrates, about 15 percent from protein, and 80 percent from fat. Note that that’s a much lower ratio of protein and a lot more fat than you’d get on other low-carb diets, but it’s this ratio that will force the body to derive much of its energy from ketones. If you eat too much protein, or too many carbs, your body will be thrown out of ketosis.

In practice, that means subsisting mainly on meats, eggs, cheese, fish, nuts, butter, oils, and vegetables — and carefully avoiding sugar, bread and other grains, beans, and even fruit. Again, if this sounds familiar, it’s because it’s not that different from the Atkins diet, among the most famous very low-carb diets that promise to get your body burning fat. (Atkins, who reportedly said ketosis is “as delightful as sunshine and sex,” promised to help people “stay thin forever,” the same way the now popular Keto Reset Dietbook promises to “burn fat forever.”)

While the evidence behind ketogenic diets for diabetes is still preliminary and the evidence for weight loss isn’t all that convincing (more on that next), the evidence of using the diet to treat epilepsy is extremely robust. The idea of treating people with epilepsy with the keto diet came about in the 1920s, when researchers observed that people who fasted experienced fewer seizures. (Researchers still aren’t sure why the diet can work for epilepsy, but a few mechanisms have been proposed, including making neurons more resilient during seizures.)

Today, studies have shown that children and adults whose epilepsy doesn’t respond to medications seem to experience a pretty large reduction in seizures when following a ketogenic diet. That doesn’t, however, mean that the diet works for other conditions.

The theory behind very low-carb diets is that they help people burn extra calories and fat — and lose more weight

Eating carbs drives up insulin production, the hypothesis suggests, stirring hunger and causing the body to hold on to fat and suppress calorie burn. But when you replace carbs with fat, you subdue hunger, boost calorie burn, and melt away fat. With fewer carbs, your body also doesn’t produce as much insulin — and that increases the rate of ketogenesis and decreases the body’s need for glucose.

This might sound great, but what’s often lost in all the boosterism is that this is still just a hypothesis. And most studies of ketogenic and other very low-carb diets suggest they don’t actually outperform others in the long run when it comes to weight loss.

Keto diets don’t seem to help people lose extra weight in the long run

When you look at head-to-head studies comparing low-carb diets to other kinds of diets, weight loss on a very low-carb diet can be a little more dramatic in the short term, but by the one-year mark, all diets perform equally miserably.

This seminal randomized trial, published in JAMA in 2007, involved 300 women and measured their weight loss on the Atkins diet compared to the Zone, Learn, and Ornish diets. The researchers found that while women on Atkins shed a few more pounds, the weight loss on low-carb diets was “likely to be at least as large as for any other dietary pattern” and “the magnitude of weight loss [on Atkins] was modest, with a mean 12-month weight loss of only 4.7 kg.” In other words, long-term weight loss on Atkins wasn’t meaningfully different from the other diets.